Document acceptor assemblies, such as those used in the vending and gaming industries, typically contain sensing means to detect the physical presence of a media being processed, or to detect the transitional state of movable elements in the machine. An effective and widely-used type of sensing means is optical sensing means, which may include a light source and a light receiver. Such sensors typically have no moving parts and do not require any physical contact with the object being sensed in order to function properly.
Document acceptors that are used for unattended payment systems, such as vending machines, are sometimes subjected to attack by various liquids, possibly as a result of fraud or vandalism to the machine itself. Another source of the hazard comes from condensation conditions which may occur when these devices are installed outdoors.
If an optical sensing device relies on a reflective surface to control and detect a light path, the presence of a liquid or film of condensation on that reflective surface may obstruct the light path and cause the sensing device to fail. One known solution to this problem involves applying a barrier coating to the optical surface. Applying a high quality mirror plating, for example, to the optical surface may maintain the effectiveness of the sensor. However, the process of applying the mirror plating can be relatively expensive and fraught with opportunities for quality control issues to arise and disrupt the machine's operation.